The present invention relates to a process for preparing highly viscous polyamides, in particular polyhexamethyleneadipamide (nylon-6,6), which are suitable for further processing in extruders to form pipes, profiles, films and the like. The relative viscosity (.eta. rel) of polyamides, measured in a 1% strength solution in 96% strength sulfuric acid at 25.degree. C., which are intended for processing by extrusion is 3.5-7.5. This corresponds to an average molecular weight (M.sub.n) of 23,000-45,000.
Known processes for preparing such polyamides by means of the postpolycondensation of granulated low-viscosity polymers in the absence of a solvent at a temperature which is lower than the melt temperature are described for example in French Pat. No. 2,143,944 which corresponds to U.S. Pat. No. 3,821,174 and also in West German Pat. No. 1,210,183 and East German Pat. No. 91,566.
Normally the postpolycondensation of polyamides in the absence of a solvent is carried out in a batchwise or continuous dryer under inert gas or in vacuo. This method makes it possible to prepare polyamides having a sufficiently high viscosity (.eta. rel up to 5.0 or more), which are suitable for further processing in the extruder. However, the postpolycondensation in the absence of a solvent has certain disadvantages.
First, owing to the low temperature, the duration of the process is too long, being in general 24 hours or more. Secondly, large amounts of protective gas which is heated to temperatures of more than 200.degree. C. are consumed. In addition, the polymer produced by postpolycondensation in the absence of a solvent has a wide molecular weight distribution on account of the nonuniform rate of removal of the water of condensation across the thickness of the granules. This in turn impairs the processing properties.
Another way of preparing highly viscous polyamides is the continuous postcondensation in the melt using various screw apparatuses.
U.S. Pat. No. 3,509,107 discloses a process for increasing the molecular weight of polyamides, for example, polyhexamethyleneadipamide. In this process, a low viscosity melt of presynthesized polyhexamethyleneadipamide (relative viscosity of an 11% strength solution in 90% strength formic acid of 17, corresponding to a molecular weight M.sub.n of 4000) is supplied to a horizontal single-screw extruder, where it is treated at 200.degree.-295.degree. C. for 0.1-1.5 hours countercurrently in an amount of 0.275-25.7 1/kg of polymer, preferably 1.28 kg-6.42 1/kg, of supplied nitrogen at 285.degree. C. The maximum relative viscosity values thus obtained are 75-77 K units, which corresponds to an average molecular weight M.sub.n of 19,000 or a relative viscosity of 3, measured in sulfuric acid.
West German Pat. No. 1,720,349 also discloses a process for increasing the molecular weight of polyhexamethyleneadipamide, by supplying the presynthesized low viscosity polyhexamethyleneadipamide having a K value of 30-60, corresponding to a molecular weight of 4000-15,000, together with 1-10% by weight of steam to a twin-screw reactor having closely meshing self-cleaning screws and a least one degassing (vent) opening. The polymer is treated at 270.degree.-285.degree. C. and a pressure of 50 torr to 2300 torr for 5-45 min. with an energy supply through the screws of 0.03 to 0.2 kWh/kg of polymer. The maximum molecular weight which is obtainable using this process corresponds to a K value of 69-72, namely M.sub.n =18,000.
The disadvantage of the abovementioned known processes for increasing the molecular weight of nylon-6,6 is that polymers having a molecular weight of more than 20,000 cannot be prepared. However, the manufacture of pipes, profiles and films requires polymers having a molecular weight of at least 23,000.
In another known process for increasing the molecular weight of polyamide, in particular polyhexamethyleneadipamide (U.S. Pat. No. 3,040,005), the polyamide in the form of a granulate, pellets or tablets, etc., is supplied to the feed funnel of a twin-screw extruder, 0.35-1.3 m.sup.3 /kg of inert gas being metered into the funnel to remove the atmospheric oxygen. On passing through the extruder the polymer melts and is processed with the cocurrently flowing inert gas which draws off the low molecular weight reaction products which are formed in the course of the polycondensation. The temperature of the polymer changes on passing through the extruder. Starting from room temperature it is raised to 296.degree. C. The total residence time is 4-8 min. The degassing of the polyamide melt is effected in vacuo. To speed up the polycondensation, the nylon-6,6 has added to it a phosphorus-containing compound, for example, sodium hypophosphite, in an amount of 0.05-1.0% by weight, based on the polymer. The average molecular weight of the polyamide (M.sub.n) rises from 17,000 to 39,000.
While this known process makes it possible to increase the molecular weight, it has the following disadvantages:
1. Oxidation of the polyamide in the vacuum treatment of the melt due to ingress of atmospheric oxygen into the extruder, since it is not possible to seal off the extruder completely.
2. High consumption of inert gas (0.35-1.3 m.sup.3 /kg) for obtaining the required molecular weight.
3. Wide molecular weight distribution.
4. Formation of gel particles. The last two disadvantages are due to excessive dewatering of the polyamide melt, caused by employing reduced pressure and/or blowing inert gas through the degassing zone of the polymer. As a result, overlong polymer chains form, and reactions between the chains (so-called superamidation) cannot take place.
5. Low output of the system and high specific energy consumption. For instance, the productivity in the case of using a twin-shaft extruder having a diameter of 31/4" (82.5 mm) was 1-2 kg/h for a specific energy consumption of 0.4-0.6 kWh/kg of polyamide.